Conventionally, this kind of apparatus includes a paper feeder as disclosed in, for example, Japanese Patent Laid-Open No. 10-95550. The paper feeder includes a paper feed tray for loading sheets of paper, a feeding part for feeding, one by one, the sheets of paper loaded on the paper feed tray, a turn guide for changing the transport direction of the sheet of paper fed from the paper feed tray by the feeding part, and a plurality of ribs formed on the turn guide so as to have long shapes along the transport direction of the sheets of paper. The sheets of paper loaded on the paper feed tray are fed one by one and the ribs on the turn guide change the transport directions of the sheets of paper.
Further, Japanese Patent Laid-Open No. 2000-95382 discloses an apparatus including a feed/transport guide for guiding a recording medium so as to bend the recording medium, a first transport roller disposed near the bending start position of the feed/transport guide, and a second transport roller disposed near an inversion start position where the recording medium having passed through the first transport roller starts to be inverted while being bent. A sheet of paper transported by the first transport roller and the second transport roller is inverted along the feed/transport guide.
Moreover, Japanese Patent Laid-Open No. 2001-233497 discloses an apparatus including a transport roller for transporting a recorded material to a recording part, a transport driven roller which is rotated in a driven manner while being pressed to the transport roller, a feed roller for feeding the recorded material to the transport roller and the transport driven roller, a supply path for supplying the recorded material while bending and inverting the recorded material along the outer surface of the feed roller, and a guide member which is separated from the outer surface of the feed roller and comes into contact with the recorded material to guide the recorded material. On the guide surface of the guide member, a plurality of guide rollers are provided which are rotated in a driven manner in response to the supply of the recorded material.
Further, Japanese Patent Laid-Open No. 2002-302316 discloses an apparatus including a holding roller for holding and transporting roll paper and a guide member disposed near the downstream side of the holding roller. The guide member has a guide surface formed to be bent opposite to the curling of the roll paper, and the curling of the roll paper is corrected by bringing the guide surface into contact with the unprinted side of the roll paper.
Moreover, Japanese Patent Laid-Open No. 2006-27799 discloses an apparatus including a transport path for transporting a recording medium to an image forming position by using a transport roller and an ejection path for ejecting the recording medium after an image is formed. The amount of transport of the recording medium is controlled by monitoring the amount of drive of the transport roller, and the amount of drive of the transport roller is corrected based on the winding angle of the recording medium relative to the transport roller and the thickness of the recording medium.
Further, Japanese Patent Laid-Open No. 2007-145539 discloses an apparatus including a recorder for recording an image on a recording medium, a storage for storing recording media of multiple sizes, a separator for separately feeding the stored recording media, and an inversion device for inverting the fed recording media. Rollers in multiple rows are disposed on the guide of the inverting device and a slider is disposed on the guide. The slider is set lower than the rollers and a recording medium such as cardboard having high stiffness is fed while inverted.
In the configurations of the prior art, a sheet of paper is sandwiched between rollers when transported or inverted and thus the rollers are in direct contact with the printed surface of the sheet. Thus when a paper transport apparatus configured according to the prior art is applied to a stencil duplicator and an inkjet printer, a printed surface is smudged.
Further, in a configuration for sucking a sheet of paper on a transport belt, the transport belt itself cannot form a curved inversion path. Thus a pair of transport apparatuses including transport belts forming linear transport paths are sequentially disposed on the front end and the rear end of a paper transport direction, and a turn guide having a curved surface for inverting a sheet of paper is disposed between the transport apparatuses. When a sheet of paper is inverted, the sheet is bent along the curved surface of the turn guide.
However, it is difficult to provide the turn guide with a sucking function and thus a suction force is not applied to a sheet of paper at the turn guide while the sheet is moved from one transport apparatus to another. Thus when a transport force is applied to the sheet at the front and rear of the turn guide, particularly cardboard having high stiffness has a large transport resistance at the turn guide, increasing the possibility that an insufficient transport force may cause faulty transport. Further, when the sheet is forcibly transported, the sheet may be curled or may be sharply bent in the worst case.
The present invention is devised to solve the foregoing problems. An object of the present invention is to provide a paper transport apparatus which can smoothly invert a sheet of paper by adjusting the curvature radius of the curved surface of a turn guide in the transport path of the sheet of paper.